The present invention relates to a power steering device which employs an electric actuator to generate an auxiliary steering force, and particularly relates to an electric power steering device that is adapted to passenger cars.
The power steering devices of the hydraulic type have heretofore been chiefly employed in the automobiles. In recent years, however, power steering devices of the electric type have been proposed in many types owing to a large variety of control functions and easiness for fitting to the vehicles. A representative example can be seen, for example, in Japanese Patent Laid-Open No. 70257/1984.
According to the conventional electric power steering device as disclosed in the above-mentioned Japanese patent publication, a so-called bias control is carried out. That is, when the operation force applied to a steering wheel reaches a predetermined value, the auxiliary steering force is increased in a stepwise manner in order to suppress the change in the auxiliary steering force when the steering wheel is being manipulated.
The electric actuator, such as an electric motor used for the electric power steering device, generates a force anytime an electric current flows thereto even when the magnitude of the electric current is so small that it will not produce an output. The electric actuator does not start the motion when the electric current supplied thereto is to small that the operation force produced is still smaller than the static friction force thereof. The operation force, however, is being produced. With the above-mentioned electric power steering device relying upon the conventional bias control, the static friction force suddenly changes in the steering system due to the rise of the electric current when the steering wheel is manipulated with a small force, producing an abnormal steering feeling for the driver.